1. Field of the Invention
The present invention relates to an optical fiber for a fiber laser device and a fiber laser device using the same, more particularly, to an optical fiber for a fiber laser device comprising a core and a cladding to transmit a high power laser beam and a fiber laser device using the same.
2. Related Art
It is demanded to develop an inexpensive light source with a high power, for the purpose of a laser beam machining application, a medical application, and the like. As to these requests, a fiber laser and an optical amplifier attract attentions, since it is possible to easily extract a single mode laser beam with a high efficiency by using the fiber laser and the optical amplifier.
FIG. 6 is a schematic block diagram of a general structure of a fiber laser now under development.
A fiber laser device 71 shown in FIG. 6 comprises a semiconductor laser (LD) 72, an exciting light combiner 73 connected to the semiconductor laser 72, an optical fiber 74 for fiber laser, a fiber grating (FBG) 75 formed at an input end of the optical fiber 74, and another fiber grating (FBG) 16 formed an output end of the optical fiber 74.
In the fiber laser device 71 shown in FIG. 6, an exciting light emitted from the semiconductor laser 72 is incident to the input end of the optical fiber 74 via the exciting light combiner 73. The optical fiber 74 for fiber laser is doped with rare earth ions (rare earth element). A plurality of the laser diodes 72 are connected to the optical fiber 74 for fiber laser to obtain a high power.
At the input end of the optical fiber 74 for fiber laser that is doped with the rare earth element, the fiber grating (FBG) 75 that has a transmittance with respect to an exciting light wavelength and a high reflective index with respect to an oscillation light wavelength is formed. At the output end on an opposite side to the input end, another fiber grating (FBG) 76 that partially reflects back the oscillation light is formed. The two fiber gratings (FBG) 75, 76 serve as a total reflection mirror and an output mirror for a laser oscillator to output a laser oscillation light L.
The fiber laser device 71 also functions as an optical amplifier, when the gratings do not form a resonator structure and a signal light having a wavelength coinciding with that of an induced emission light is superimposed on the exciting light to be propagated.
As the optical fiber 74 for fiber laser that is doped with the rare earth ions, a double clad type optical fiber as shown in FIG. 7 is generally used. A core region 77 of the double clad type optical fiber 74 is doped with the rare earth element such as Nd, Yb, Er, Th or the like. A cladding comprises a first cladding region 78 having a refractive index lower than that of the core region 77, and a second cladding region 79 having a refractive index lower than that of the first cladding region 78. Further, a coating layer (not shown) comprising a material generally used in the art or the like is provided at an outer periphery of the second cladding region 79.
An exciting light Le is propagated in multimode through the first cladding region 78 and gradually absorbed by the core region 77 located at a center of the optical fiber 74, so that the exciting light Le is attenuated. According to an end-face excitation type fiber laser using such a double clad structure optical fiber, conversion efficiency from the exciting light Le to a laser oscillation light L is high. The output of the end-face oscillation type fiber laser has been increased year by year, and an oscillation of 10 kW class was also realized.
Further, as conventional optical fibers, JP-T-2002-541507 discloses a photonic crystal fiber and a method for its production, and JP-A-2000-35521 discloses an optical fiber composed of a hollow hole, a cladding having a diffraction grating.
As described above, the fiber laser has a superior beam quality. Further, the output power has bee increased recently. In addition, the realization of the fiber laser with 10 kW class was achieved. Therefore, the development of the fiber laser raises expectation for various processing applications, particularly, application for a laser beam oscillation source for welding or cutting of metallic members. As a problem for putting the high power fiber laser to practical use, there is an issue of safety measures.
In particular, a laser light blocking function in case of fiber disconnection is an essential and indispensable function for the fiber laser device. In the event that the optical fiber is broken, a laser beam having a high energy density may be emitted from a disconnection point, thereby damaging articles around the disconnection point. A system for detecting the disconnection of a fiber laser main body or an optical fiber for fiber laser is generally introduced to avoid such an accident.
To be more concrete, a cable structure incorporating the optical fiber for fiber laser is designed for detecting the disconnection of the fiber laser main body or the optical fiber, instead of designing the fiber laser main body.
For example, FIG. 8 is a schematic diagram of a fiber laser disconnection detection cable 81. In the fiber laser disconnection detection cable 81 as shown in FIG. 8, a metal tape 83 is provided to surround an optical fiber 82 for fiber laser in a cable jacket 84, and an electric current is flown through the metal tape 83. When the optical fiber 82 for fiber laser is disconnected, the laser beam leaks from the optical fiber 82, so that the metal tape 83 will be broken by the leaked laser beam. By detecting the disconnection of the metal tape 83, it is possible to detect the disconnection of the optical fiber 82 for fiber laser.
However, according to this structure, it is necessary to flexibly incorporate the metal tape 83 between the optical fiber 82 for fiber laser and the cable jacket 84. In addition, it is necessary to provide an insulation processing for suppressing a short circuit between the metal tapes 83 even if the metal tapes 83 contact with each other. Accordingly, there is a disadvantage in that manufacturing of such a cable structure is difficult and requires complicated steps.
There are further disadvantages in that a total configuration of the cable is complicated since it is necessary to provide the metal tape 83 at an outer periphery of the optical fiber 82 for fiber laser, and that it is hard to install a fiber laser transmission system cable, since an outer diameter of the cable as well as a weight of the cable are increased.